The incorporation of phyllosilicate fillers in polymer films is used to enhance mechanical properties, heat resistance, and enhance barrier properties. Other properties improved with fillers include improved chemical resistance, electrical resistance, and flame retardancy. Since the late 1980s a great deal of research around the world has focused on methods to incorporate fully exfoliated smectite clays, primarily montmorillonite due to its extremely high aspect ratio, into a variety of thermoset and thermoplastic polymers.
Thermoplastic polymers such as polyethylene are used in flexible packaging applications because of its high resistance to the diffusion of water vapor and other polar molecules. When applied to a paper substrate the polymer film provides strength, and improved barrier properties. However, laminate structures such as these are difficult to recycle due to the tight bond between the polymer and the paper substrate. Alternative approaches include the use of wax coatings to impart water barrier resistance, especially water vapor, and still maintain recyclability. Unfortunately, wax coatings are poor barriers to nonpolar gases like oxygen and carbon dioxide, and the wax surfaces have a tendency to be slippery.
Because their surfaces are hydrophilic, the smectite clays must be surface-treated to render them compatible with olefinic materials. The approach that has most often been used is based on the technology utilized for the last fifty years to make organoclays as rheological control agents in paints, inks, greases, etc. This approach utilizes quaternary amine-based surfactants to render the basal surface of the clay compatible with the olefinic matrix. Various high-molecular-weight quaternary ammonium salts have been used such as dimethyl dihydrogenated tallow ammonium chloride, dimethyl benzyl hydrogenated tallow ammonium chloride, and methyl benzyl dihydrogenated tallow ammonium chloride. Other onium ions that have been used include the phosphonium and sulfonium groups. Surprisingly, this approach has not been very successful in promoting clay exfoliation in olefinic polymers such as polyethylene and polypropylene and their copolymers.
Methods of producing organoclays that incorporate high-molecular-weight hydrotropes on the basal surface and an edge modifying surfactant adsorbed along its edges to render the clay platelets edges organophilic, yield organoclays which readily disperse in polyolefin systems.
Wax nanocomposites have many potential applications in paper packaging like increasing barrier properties (i.e., oxygen and moisture vapor transmission, oil and grease resistance, water resistance), increasing strength, increasing coefficient of friction, increasing block resistance, and increasing printability. With improved barrier capabilities and chemical resistance, wax nanocomposites could find use as industrial coatings and protective packaging, corrugated containers (i.e., meat, cheese and poultry packaging, flower shipping boxes), coated folding cartons (i.e., detergent boxes, dairy and bakery products, frozen food, pet food), bags and wrapping papers (i.e., fast foods, microwave packaging, pet foods, cement bags), industrial packaging (i.e., ream wrap, roll wrappers, beverage carriers), and liquid packaging (milk; juice, oil). The problem of recycling polymer-laminated paper is solved by using wax-based, emulsion nanocomposites in place of extruded polymer films. In addition, the coating speed of emulsion systems is considerably faster than polymer extrusion coating thereby reducing manufacturing costs.
Prior art has focused on the incorporation of nanoclays into waxes for improvement of various properties including reduced shrinkage and cracking of the wax when it is cooled from the melt and increasing melt viscosity. There is however, no evidence in the scientific literature to suggest that the incorporation of organoclays into waxes would lead to increased barrier properties of wax films and no mention of this application is found in prior art. This is consistent with polyolefin systems, where there is also no prior art demonstrating improved barrier performance in polyolefin nanocomposites. This is puzzling since the exfoliation of an organoclay is generally expected to reduce gas permeability by virtue of increasing diffusion path length. Because the clay crystal lattice is impermeable, anisotropic orientation of the clay platelets would lead to a tortuous diffusion path. In fact, the addition of organoclays, according to prior art, actually results in a decrease of barrier properties.
Thus a need exists for a method of improving the barrier properties and chemical resistance of wax and wax/polymer coatings while at the same time improving certain physical attributes such as surface friction and maintaining flexibility.